Dna or rna viruses

Viruses are very interesting in that they can only survive inside a living cell. So they must have a living cell in order to survive and replicate. Antibiotics are not effective against viruses, but vaccines are, as well as some antivirals. Bettie J. Graham, Ph. Thus these two strategies are closely related. Some ambisense viruses package copy genomes that can be used as templates for transcription, such that the full complement of viral genes can be transcribed soon after infection.

It should be noted that packaged copy genomes are not mRNAs and are not translated. Reoviruses also have segmented genomes, packaging 11—12 segments of dsRNA.

Reoviruses are nonenveloped and particles consist of two or three concentric icosahedral capsid layers. A unique feature of the reovirus replication cycle is that the genome segments are transcribed from within the capsid.

The genomes of RNA viruses have some common general features. Obviously there are one or more open reading frames that encode the viral proteins. But there are also regions of RNA that do not code for protein. These non-coding regions NCRs or untranslated regions UTRs are often highly conserved within a virus family, indicating that they have important functions. NCRs may have specific, critical nucleotide sequences but in some cases they are regions of the genome that fold into conserved structures, and structure may be more critical than a specific sequence.

Of course a source of RdRp must be supplied. RdRp may be encoded in the minigenome or may be supplied in trans by using a cell line stably expressing the viral RdRp, for example. The sequences required to direct RNA replication are often fairly simple and can be linked to virtually any RNA sequence to drive its replication. These promoter sequences can be rather short but provide a means to direct the RdRp to internal sites on the genome. There may also be specific RNA sequences that signal polyadenylation.

There are a variety of different strategies that RNA viruses use to regulate transcription and genome replication, but all involve RNA sequences found in the genome.

The RNA genomes of some viruses are highly structured and extensively base paired. The IRES serves as a platform for ribosome assembly. Promoters can be quite long and complex and promoter regions themselves are not transcribed. It is particularly important, in the case of genome synthesis, that genetic information not be lost or modified; however, mRNAs are often capped and polyadenylated.

Are the methods for priming viral mRNA synthesis the same or different from the methods of priming genome replication? The RNA viruses seem to have experimented widely. For example, the picornaviruses use poly A tracts encoded in the genome.

Among the negative-strand RNA viruses, those in the order Mononegavirales use a stuttering mechanism to synthesize long poly A tracts from short poly U tracts Fig. A strategy to regulate mRNA synthesis. This figure shows the organization of a paramyxovirus genome paramyxoviruses are members of the order Mononegavirales ; negative-strand RNA viruses with unsegmented genomes. Each protein-coding region is flanked by regulatory sequences that control capping and polyadenylation.

The order of the genes on the genome regulates the relative quantities of mRNAs synthesized. Because RdRp does often dissociate from the genome during transcription, the downstream genes are produced in lower quantities. Even with fairly simple genomes, RNA viruses must, and do, regulate the amounts of genome, copy genome, and mRNAs that are synthesized during an infection.

It is much more efficient to synthesize many genomes from each copy genome. Internal promoters for mRNA synthesis can vary in sequence, controlling the relative affinity of the transcription complex for each mRNA.

An important feature of RNA viruses is that many exist in nature as quasispecies. The term quasispecies is used to describe a group of closely related, but nonidentical genomes Fig.

A Positive-strand RNA viruses exist as quasispecies, complex mixtures of related genomes. Once the payment is made, you need to log in to the Downloads Page to download the files check your email for the login details. Subscribe to our newsletter and never miss an important update! The viruses are non-cellular organisms that are characterized by having an inert crystalline structure outside the living cell.

Viruses are obligate parasites. Once they infect a cell, they take over the machinery of the host cell to replicate themselves , killing the host.

The name virus that means venom or poisonous fluid was given by Pasteur. The protein coat called capsid made of small subunits called capsomeres protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms.

AIDS is also caused by a virus. Viroids Viroids are infectious agents that are smaller than viruses. A viroid is a free RNA, it lacks the protein coat that is found in viruses , hence the name viroid. The RNA of the viroid was of low molecular weight. Viroids cause potato spindle tuber disease. Virus Viroid It is a nucleoprotein particle. It is an RNA Particle. Viroid is formed of only RNA.

A protein covering of coat is present. A protein coat is absent. Virus has a larger size. Viroid has a smaller size. Virus infects all types of organisms. Figure 2 Figure 2. Figure 3 Figure 3. Evolution Changes in pyramid level might be mediated by virus evolution or changes in virus ecology Transmission Demonstrating that an infected human has the potential to transmit the infection to another human is not always straightforward. Phylogenetic Analysis One approach to resolving the question of human-to-human transmission is analysis of nucleotide sequence data, sometimes referred to as forensic phylogenetics.

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